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Research into the oxygen pressure leaching of zinc sulphide concentrates has identified that oxygen absorption is a rate-controlling process. In this study, the transfer of oxygen from air to aqueous sodium sulphite solutions by gas pumping impellers was studied as a simulation of the gas-liquid mass transfer step.
A 200 litre right-cylindrical lucite mixing vessel with near-hemispherical bottom was employed. With this tank design, interference with impeller discharge is minimized and the environment approaches the ideal case of an impeller in the ocean. Four radially mounted baffles could be inserted to induce flow conditions characteristic of industrial mixing vessels. The effects of baffles and impeller type, diameter, immersion depth, and tip speed were evaluated in terms of the oxygen transfer rate, power consumption, and power efficiency of oxygen transfer.
It was demonstrated that in the absence of baffling a critical tip speed exists, above which the impeller begins to pump gas at a rate proportional to the impeller tip speed. This critical tip speed is linked directly to the depth of immersion of the impeller on the basis of a theoretical energy balance at the impeller tip. The fit of experimental data to the theoretical was explained in terms of the geometry of the impeller and the shape of the gas vortex it produces. When baffling is introduced, the results obey the critical tip speed relation less accurately, as baffle-impeller interactions significantly alter the flow patterns and change the gas pumping mechanism to bubble capture from surface eddies. In the unbaffled vessel, axial flow impellers pump oxygen at lower rates and energy efficiencies than radial disc impellers, unless placed at shallow immersion. Gas pumping by dual impellers offers no advantage over a single radial disc impeller operating at the lower immersion depth.
At a given impeller speed, oxygen transfer is increased by the addition of baffles, but power consumption increases at a faster rate so that the energy efficiency is lower than without baffles. However, it was possible with impellers of smaller diameter at a shallow immersion depth to sustain a vortex in the baffled vessel, which gave the largest rates and efficiencies of oxygen transfer. This suggests it is possible to reduce the degree of baffling, and that the rate and efficiency of oxygen transfer can be optimized through judicious selection of impeller diameter and baffle size.

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